In bottles of polyethylene terephthalate manufactured by stretching blow molding, when the bottle is filled with contents at a high temperature, a lip portion of the bottle often becomes deformed. It is said that such a deformation results from the fact that a parison is subjected to stretching blow molding under the condition that a neck of the parison being injection molded is held, and therefore, the parison neck is not stretched but remains injection molded, the parison neck being deformed due to heating.
In view of the foregoing, an attempt has been made by the present inventor to mold a neck of a parison using a resin having a heat resistance, for example, such as polycarbonate, in order to impart thermal stability to a lip portion of the bottle. A molding method in this case is carried out by making use of an insert molding generally used, wherein a neck of a parison, which has been pre-injection molded, is held by a neck mold, the neck is inserted into a mold for molding a main parison portion to mold the main parison portion and at the same time, they are integrally connected to form a composite product.
In the case of this insert, pre-molded parison necks have to be continuously supplied one by one to neck molds, and holding of the parison necks by the neck molds always has to be carried out with accuracy.
However, it is extremely difficult to positively and continuously supply parison necks one by one to a plurality of neck molds, and it is more difficult to perform accurate holding of parison necks by neck molds. An insert device of high precision is required.
As a result of further repeated studies, the present inventor has found that it is advantageous in terms of manufacture as well as heat resisting bottles themselves to employ a process wherein a main parison portion provided with an outer layer of a parison neck and in inner layer of the parison neck is continuously molded within the same apparatus to thereby mold a composite parison in which the outer layer of the parison neck comprises the other heat-resisting synthetic resin, rather than a process wherein a pre-molded parison neck is inserted to mold a composite parison in which the neck comprises inner and outer (i.e., two) layers.